So far, a rotation angle detector (referred to as a rotation sensor hereinafter) is used in a variety of field, such as an engine for vehicle, a steering wheel, a DC motor, or the like. For example, the rotation sensor is used in a case of attaching the same to a shaft to be rotated, such as a steering shaft or the like in a vehicle, and then of detecting a rotation angle of a steering wheel to be integrated with such the shaft.
However, in a case of rotating a rotation body, such as the steering wheel or the like, regarding an absolute rotation angle as a θ, a state of the rotation body is similar from a visual point of view for between zero degree<0≦360 degrees and 360 degrees<0≦720 degrees, and then it is difficult to distinguish therebetween. Therefore, there is proposed a rotation sensor for detecting the absolute rotation angle as the θ in the case of rotating the rotation body.
According to a patent document 1, there is proposed a rotation sensor, which comprises: a first detectable body to be rotated due to a rotation body to be measured; a first rotation detecting unit for detecting a signal of the first detectable body; a second detectable body to be rotated due to a rotation body or to the first detectable body; a second rotation detecting unit for detecting a signal of the second detectable body; and an absolute angle detecting unit for detecting an absolute rotation angle of the rotation body, with reference to a gradient of a signal regarding a signal to be output from the second rotation detecting unit and to the values of a voltage (three values of zero, a positive value and a negative value).    [Patent Document 1] Japanese Patent Application Publication No. 2002-098522